Composite keys are widely used in many circumstances such as the word processing situation or the game playing situation. When plural keys of the composite key are simultaneously depressed, a specified function can be implemented. However, during the process of depressing the composite key, some problems may occur. For example, after two keys are depressed, the action of depressing the third key does not output any signal. This problem is also referred as a ghosting problem.
Conventionally, the keys of the keyboard are arranged in a keyboard matrix. When one key is depressed, a keyboard controller realizes the information of the depressed key according to the column information and the row information and outputs the corresponding signal. Generally, the intersection between each row and each column of the keyboard matrix represents a corresponding key. When one key is triggered, the keyboard starts to scan the keyboard matrix and recognizes which key is depressed. For example, the keyboard controller scans all columns of the keyboard matrix sequentially. If the corresponding signal is received from a specified row when a specified column is scanned by the keyboard controller, the keyboard controller can realize which key is depressed according to the column information and the row information of the closed loop.
The minimum rectangle unit of the keyboard matrix is composed of four keys. When any of the four keys is depressed, the signal can be successfully generated. When two of the four keys are arbitrarily depressed, the keyboard controller can realize which keys are triggered. However, when three keys are simultaneously depressed, two row information and two column information are transmitted to the keyboard controller. Since four keys are defined by two rows and two columns, the keyboard controller cannot recognize the three depressed keys from the four keys according to the row information and the column information only. Under this circumstance, the fourth key is erroneously judged as the on-state key. That is, the fourth key is referred as a ghost key.
When the ghost key exists, the keyboard controller cannot accurately judge which keys are depressed to generate the misjudged signals. For preventing the erroneous signal from being sent out, the keyboard controller may ignore the signal that is generated in response to the depressed third key. That is, only the signals corresponding to the earlier two keys are sent out. As mentioned above, if three keys at three of every four adjacent locations of the same matrix are simultaneously depressed, the ghosting problem occurs.
Since a membrane keyboard uses the keyboard matrix to judge the depressed key, the ghosting problem occurs. For avoiding the ghosting problem, the membrane keyboard is additionally equipped with a control IC that is implemented with a complex programmable logic device (CPLD). In addition, the key that is easily formed as the ghost key (e.g., one key of the composite key) has a novel circuitry design for avoiding occurrence of the ghost key. However, the above approaches need to develop the new design and cannot be directly applied to the commercially available membrane keyboard. Consequently, the cost and time in development are increased.
Therefore, there is a need of providing an improved membrane keyboard with a novel design to avoid the ghosting problem